The proportion of mechanical pulps used as raw material in the manufacturing of paper has been rapidly increasing over recent years. The competitive quality of mechanical pulps was mainly created by thermomechanical pulp which is produced by attrition-refining wood splinters between two refiner disks. In the preparation of thermomechanical pulp, an elevated temperature is used in order that the lignin might soften and the fibers might separate without being ruptured. In addition, fiber friction work takes place between the refiner disks, and this favorably fibrillates the fibers, together with the fiber/grinder disk friction work. This fibrillation of fibers endows the paper made of such pulp stock with greater strength than does the fiber produced in cold attrition refining or in stone refining. This manufacturing method has been known in principle for a long time. Only in the most recent years has the procedure begun to be more widely adopted. The causes of such development have been, in the first place, the increase in price of the wood raw material and the technical development of the thermomechanical pulp producing equipment.
While the thermomechanical pulp technology has been rapidly developing, attention has also begun to be paid to the high consumption of electricity in this process. A typical thermomechanical pulp installation consumes about 1800 to 2000 kWh of electric energy per ton of pulp. Since evidently the efficiency of the difibration itself is below 1%, virtually all of the shaft power that is put in has converted to heat. But, as it is generally judged, heat is no practical utility in itself; its usability depends on how far its temperature exceeds that of the next source of heat with abundant availability.
It is generally stated regarding thermomechanical pulp processes that as soon as the temperature rises over a certain limit, for instance 127.degree. C. for spruce, the lignin will soften excessively and spread as a molten product over the surface of the fiber material that is being ground, thereby preventing the formation in the paper manufacturing process of the hydrogen bridges, which are typical of cellulose. Owing to this, in favorably operating thermomechanical pulp installations the temperature and the equivalent saturated steam pressure are controlled so such that this temperature is not surpassed. A typically favorable preheating and refining temperature actually used is about 120.degree. C. and the equivalent steam pressure is, about 200 kPa.
However, these temperatures and pressures mentioned do not yet enable the usability of said steam in typical steam consumption applications. It is, naturally, always possible to compress steam of an arbitrary pressure to any given pressure and temperature, but this involves separate extra costs. Heat has been recovered from steams with pressure as mentioned, by contacting the steam with water, whereby hot water is obtained. But as a rule hot water of 70.degree. to 90.degree. C. is a commodity found in abundance in paper mills.
The heat recovery systems of many thermomechanical pulp processes have moreover been encumbered by the particular drawback that the steam obtained either directly from the refiner or from the refiner through a preheater contains large amounts of air, in fact typically 10% by volume. If it is contemplated, e.g. out of corrosion considerations, to heat exchange this raw steam for pure steam, major difficulties will be experienced in the heat exchange process owing to the low condensation heat transfer coefficient due to the air content.